Advances in internet usage are leading to bandwidth demand increase in all sections of the network. One such advance has been in the wireless local area network (LAN) area. Demand for wireless LAN has experienced phenomenal growth. This demand has been driven by users connecting notebook computers to networks at work or at mobile gathering places, among others. Growth has extended beyond the PC as well. Consumer applications like music streaming, internet telephony, gaming and in-home video transmission are also fueling growth in bandwidth.
These demand increases on wireless LAN have spurred extensive standards development in the technical area. Several wireless communications standards such as the Institute of Electrical Engineers (IEEE) 802.11 standard have emerged. IEEE 802.11 denotes a set of wireless local area network (WLAN) air interface standards for short-range communications ranging from tens of meters to a few hundred meters. One such WLAN standard is 802.11b. This standard specifies raw data rates up to 11 Mbps using modulation techniques of Complementary Code Key (CCK) and/or Direct-Sequence Spread Spectrum (DSSS). The 802.11a standard, defined contemporaneously with 802.11b, uses a more efficient method of transmission called Orthogonal Frequency Division Multiplexing (OFDM). The 802.11a standard enabled data rates up to 54 mbps, but due to incompatible radio frequency band of 5 GHz, as compared to 2.4 GHz for 802.11b, this standard was not widely deployed. In mid-year 2003, IEEE ratified 802.11g, which applied OFDM modulation to the 2.4 GHz band. Much of WLAN client hardware supported both 802.11a and 802.11g.
The follow on generation in the development of standards is 802.11n. The 802.11n standard provides for a variety of optional modes that dictate different maximum rates. The standard allows manufacturers to tune capabilities to offer different price points and performance. The 802.11n standard offers raw data rates up to 600 Mbps, while a 300 Mbps speed device could also be built consistent with 802.11n specification.
The 802.11n standard improves OFDM implementation using a higher maximum code rate and higher bandwidth. It improves the raw rate from 54 Mbps to 65 Mbps. Further, one of the widely known components of the standard is a Multiple Input Multiple Output or MIMO. MIMO exploits a radio phenomenon called multi-path. Multi-path involves transmitted information bouncing off doors, walls and other objects. This information reaches the receiving antenna multiple times through different paths and at slightly different times.
Multi-path degrades wireless performance if it is not controlled. MIMO technology, adopted in 802.11n standard, usefully deploys multi-path through space division multiplexing (SDMA). The WLAN transmitter device splits the data stream into multiple parts, called spatial streams. Each spatial stream is transmitted through separate antennas to corresponding antennas on the receiver. The 802.11n supports up to 4 spatial streams. While doubling or quadrupling the spatial stream leads to increase in raw data rates, the cost and power also tend to increase due to increased processing required per antenna pair. A MIMO system is characterized by the number of transmitter antennas by the number of receiver antennas. A 4×4 MIMO, for example, has four antennas on the transmitter and 4 antennas on the receiver.
MIMO performance can be improved by beam-forming and diversity. Beam-forming directs the radio signal on to the target antenna. This improves range and performance by limiting interference. Diversity exploits multiple antennas by combining the outputs of or selecting the best subset of a larger number of antennas than required to receive a certain number of spatial streams. Excess antennas may be used to say combine multiple received streams to one stream, operating over longer range. Similar trade offs may be made for increasing raw data rates, with a fixed range.
The 802.11n standard, in summary, advances wireless LAN (WLAN) through better OFDM characteristics, space division multiplexing through MIMO, diversity, power saving methods, doubling of channel from 20 MHz to 40 MHz, MAC level aggregation of overhead, and reduced inter frame space.
In the follow on standards, referred to as 802.11 for Very High Throughput (VHT) at 5 GHz band, the RF bandwidths targeted are up to 160 MHz and data rates are up to 6.933 Gbps. More efficient signal processing schemes are being deployed to reduce noise and improve the signal to noise ratio. Traditionally, pilot tones in the data symbols have been used to perform phase tracking during data symbols, but for 802.11n and above generations, this is compute costly and not feasible during MIMO channel estimation. The pilot tones as defined in 802.11n for the Long Training Fields (LTFs) vary from stream to stream and therefore cannot be used for accurate phase tracking.
For a 5 GHz carrier frequency, a 2 parts per million drift amounts to a frequency drift of 100 KHz. This frequency drift, with 4 symbols, amounts to a phase rotation of 5 degrees. For eight symbols, this doubles to 10 degrees. In OFDM, since the signal is carried in the phase, a phase drift leads to lower signal to noise ratio. This phenomenon makes the wireless network degrade on both performance and throughput.